Your post made me laugh, my husband decided one year to surprise me with a new stove/oven 3 days before Thanksgiving. With, "Oh by the way, if we have the wrong gas connection they won't be able to install it." But wait they are taking the old one away? Yes... , It took everything I had not to let my thoughts fall out of my mouth because we really did need a new stove and he really thought he had done this wonderful thing for me. Luckily they were able to install it and I was able to cook Thanksgiving dinner. And a sewing machine is as personal as a stove in the kitchen. Which may sound funny but buying the wrong one can be a major disappointment. So the idea of taking her to a few stores to get an idea of what she truly wants may not be a bad thing.
If she really wants a Featherweight - they are wonderful little machines - I have 2 and love them both. But I have my 12-91 that belonged to my mother which is a full sized machine and so is my Janome 6600P. I know several women that use their FW as their main machine due to lack of space. This is the 160th year of Singer so they came out with a Special Featherweight and it has more stitches. It will be a lot more than a vintage machine unless you want one that has been totally refurbished. A 221 black vintage Singer may or may not be an investment, but if it is what she wants and you give it to her, she will never part with it so the future value is nothing to worry about.
Good luck!